Ongoing studies of the organization and expression of endogenous xenotropic MuLV DNA sequences have highlighted the need for cloning each of the 28 defined HindIII restriction fragments found in BALB/c mice. The availability of such a clone library would allow us to better define the role in virus expression repression and tumor induction of each proviral MuLV sequence along with its flanking DNA. We used the cosmid system to begin our cloning attempts. Preliminary data suggested we had cloned a number of xenotropic proviral DNA sequences including the most prominent 11.6 kb fragment. This fragment is not only found in BALB/c mice but is common to most mice tested. However, every attempt to amplify the various primary cosmid clones resulted in all xenotropic env reactive insert fragments being lost. We then tried cloning the various BALB/c HindIII restriction fragments in the Charon 20 vector. The efficiency of this vector was too low and we were again unable to successfully clone any appropriate fragment. We have now initiated studies using the LambdaJ-1 vector. Thus far, HindIII arms which can be shown to easily ligated have been prepared as well as concentrated fractions of appropriately sized HindIII restriction fragments from somatic cell hybrid DNA. Attempts to isolate and characterize appropriate clones continues.